Wide bandgap transistor devices are being used with increasing frequency due to their performance advantages in high power, high temperature, and high frequency applications. While the advantages of wide bandgap transistor devices are well known, wide bandgap materials systems have been slow to replace their conventional counterparts due to challenges encountered in the design and manufacture of wide bandgap devices. One specific challenge facing many wide bandgap transistor devices used in high frequency applications is reduced performance due to a capacitance between two or more electrodes of the device. For example, in the case of field-effect devices and high electron mobility transistors (HEMTs), capacitive coupling between a gate electrode and a drain electrode reduces both the gain and breakdown voltage of the device.
In recent years, field plates have gained traction as a means for mitigating the negative effects of undesirable capacitances between electrodes of wide bandgap transistor devices. A field plate is a conductive plate that is placed over a portion of a charge transport layer between two electrodes in the transistor device such that the field plate is electrically isolated from the charge transport layer and the electrodes. Generally, field plates are used in field-effect devices and thus are placed between a gate and a drain electrode of the device, however, many different transistor devices may benefit from the use of field plates. The field plate reduces the capacitance between the gate and drain electrodes and redistributes an electric field on the drain side of the device in order to improve the break down voltage, gain, and maximum operating frequency of the device.
While field plates have allowed wide bandgap transistor devices to achieve performance metrics that were previously unheard of, many field plates continue to suffer from relatively high electromigration of one or more conductive materials within the field plate. Electromigration is the movement or transport of material due to a transfer of momentum between conducting electrons and one or more atoms in the migrating material. In other words, electromigration is the physical movement of a material over time due to a current flowing through the material. In the case of field plates, electromigration may cause undesirable connections between the field plate and an electrode or charge transport layer due to the migration of conductive material from the field plate through one or more spacer layers. Additionally, electromigration may cause disconnection of a desired connection between the field plate and one or more electrodes or voltage sources due to the migration of conductive material away from the desired connection points. Accordingly, electromigration of the field plate may cause the field plate to cease functioning, reduced performance of the transistor device, and even complete failure of the transistor device.
In light of the discussion above, there is a present need for field plates with reduced electromigration characteristics and methods for manufacturing the same.